1. Field of the Invention
This invention relates to a method for growing improved quality devices using low temperature magnesium doped nitride films.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for the fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques comprising molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
Nitride based optoelectronic devices began their quick ascent to commercialization with the advent of the use of a thin nucleation layer prior to the deposition of high quality GaN [1,2]. This technique is employed due to the lack of a native substrate available for GaN growth. More recently, techniques such as the development of p-type GaN by magnesium doping followed by high temperature annealing have also proved vital.
However, the development of indium gallium nitride (InGaN) as the active layer for short wavelength devices enabled nitride based light emitting diodes (LEDs) and laser diodes (LDs) to overtake many other research ventures. Consequently, InGaN has become the dominant material system used for visible light semiconductor applications.
FIG. 1 illustrates a typical deposition temperature profile as a function of deposition time for fabricating a nitride based diode device. Most nitride LED and LD processes using MOCVD begin by heating the substrate to a temperature of approximately 1050° C. for 5-30 minutes (referred to as the “Bake” step in FIG. 1). This initial step is believed to aid in the removal of any impurities that might be present on the surface of the sapphire (Al2O3) substrate and substrate holder.
The temperature is then lowered to between 450-700° C. to grow the low temperature GaN nucleation layer (NL) (referred to as the “NL” step in FIG. 1). Most nucleation layers are deposited to a thickness of approximately 10-50 nm.
Once a desired nucleation layer thickness is achieved, the substrate temperature is increased to approximately 1050° C. for the deposition of high quality GaN thin films (referred to as the “GaN:Si” step in FIG. 1). This GaN film can be doped with silicon (Si) to achieve n-type conductivity for the electrically negatively charged material because of the over abundance of electrically active electrons that are present.
Once the n-type GaN:Si is deposited, the substrate temperature is decreased to deposit the InGaN multiple quantum well (MQW) (referred to as the “MQW” step in FIG. 1). The use of InGaN as the active region for Nitride based semiconductors was first developed by Nakamura et al. [3]. Reference [3] manufactured an InGaN/AlGaN double heterostructure (DH) LED, and initial results showed such LEDs could produce 1 candela (cd) in brightness [3]. This structure was later modified to a multiple period InGaN/GaN quantum well, or multi quantum well (MQW), which resulted in even higher efficiencies including internal quantum efficiencies reaching an estimated 70%. It was also discovered that indium incorporation is highly dependent on the growth temperature of the film. Consequently, different indium compositions can be attained by varying the growth temperature of the InGaN film. In turn, this changes the energy gap of the active material and leads to a variation in the emission wavelength when the film is biased or excited by photoluminescence. Typical substrate temperatures for InGaN deposition range from 700-900° C. and are dependent on growth conditions and reactor geometry. Above this temperature range, indium nitride (InN) becomes volatile and easily dissociates leading to low InN incorporation in the films. Below this temperature, InN incorporation greatly increases which can lead to indium clustering and poor film quality.
While maintaining the same temperature used to grow the MQW, an AlGaN electron blocking layer is usually deposited on top of the MQW. Typical thicknesses for this layer range from 5 nm to 300 nm. The AlGaN film acts as an electron blocking layer when the LED is biased. This is due to the larger bandgap energy of the AlGaN present in the layer, wherein the AlGaN layer acts as a potential energy barrier that the electrons must overcome, thereby aiding in the confinement of the electrons to the active region of the device. This confinement increases the probability of radiative recombination in the active region of the LED [4].
The film is then heated to a substrate temperature between 1000° C. to 1100° C. in order to deposit a film of p-type GaN doped with magnesium (Mg) (referred to as the “GaN:Mg” step in FIG. 1). Typical thicknesses for the Mg doped GaN films range from 150 nm to 500 nm. Mg incorporation into GaN has been shown to act as a deep level acceptor, causing the Mg doped nitride material to have a lack of electrons which results in the film having an electrically positive behavior (p-type GaN).
For many years, it was believed that a p-type GaN film was not possible, until Amano et al [8] discovered that p-type GaN was possible by doping the GaN film with Mg. However, in order for the Mg doped GaN film to behave like a p-type material, the film had to undergo a Low-Energy Electron-Beam radiation (LEEBI) treatment after growth [5]. Nakamura et al. later discovered that a p-type film could also be obtained by annealing the Mg doped GaN film at temperatures greater than 600° C. in nitrogen (N2) after growth [6,7].
Although Mg doped GaN has been extensively used in nitride based LEDs, the use comprises of GaN films grown at temperatures higher than the deposition temperature of the preceding InGaN MQWs. As mentioned previously, InN has a high volatility and readily evaporates out of the InGaN films when exposed to a high enough temperature and/or a low temperature for an extended period of time. This time and temperature value is commonly referred to as the material's thermal budget.
The present invention distinguishes itself from the above-mentioned methods by the use of a low temperature (LT) Mg doped nitride layer in order to improve the quality of diodes and devices comprising InN. As a result, there is a need for improved methods for the growth of LT Mg doped nitride planar films, wherein the thermal budget of the previously deposited InN containing MQW is considerably reduced. The present invention satisfies this need.